The present invention relates to automated workpiece handling systems, and more particularly, to methods and devices for aligning a cassette for workpieces in an automated workpiece handling system.
In order to decrease contamination and to enhance throughput, semiconductor processing systems often utilize one or more robots to transfer semiconductor wafers, substrates and other workpieces between a number of different vacuum chambers which perform a variety of tasks. An article entitled xe2x80x9cDry Etching Systems: Gearing Up for Larger Wafersxe2x80x9d, in the October, 1985 issue of Semiconductor International magazine, pages 48-60, describes a four-chamber dry etching system in which a robot housed in a pentagonal-shaped mainframe serves four plasma etching chambers and a loadlock chamber mounted on the robot housing. In order to increase throughput, it has been proposed to utilize two loadlock chambers as described in U.S. Pat. No. 5,186,718. In such a two-loadlock chamber system, both loadlock chambers are loaded with full cassettes of unprocessed wafers. FIG. 1 of the present application illustrates two typical loadlock chambers LLA and LLB, each having a cassette 190 therein for holding unprocessed wafers 192 to be unloaded by a robot 194 in a transfer chamber 195 and transferred to various processing chambers 196 attached to a mainframe 198.
The loadlock chamber LLA, for example, is a pressure-tight enclosure which is coupled to the periphery of the mainframe 198 by interlocking seals which permit the loadlock chamber to be removed and reattached to the mainframe as needed. The cassette 190 is loaded into the loadlock chamber LLA through a rear door, which is closed in a pressure-tight seal. The wafers are transferred between the mainframe 198 and the loadlock chamber LLA through a passageway 199 which may be closed by a slit valve to isolate the loadlock chamber volume from the mainframe volume.
As shown in FIG. 2, a typical cassette 190 is supported by a platform 200 of a cassette handler system 208, which includes an elevator 210, which elevates the platform 200 and the cassette 190. The platform 200 has a top surface, which defines a base plane 220 on which the cassette 190 rests. As the cassette includes a plurality of xe2x80x9cslotsxe2x80x9d 204 or wafer support locations, the elevator moves the cassette to sequentially position each of the slots with the slit valves to allow a robot blade to pass from the mainframe, through the slit valve, and to a location to xe2x80x9cpickxe2x80x9d or deposit a wafer in a wafer slot.
The slots 204 of the cassette may be initially loaded with unprocessed wafers or other workpieces before the cassette is loaded into the loadlock chamber LLA. The number of unprocessed wafers initially loaded into the cassette may depend upon the design of the cassette. For example, some cassettes may have slots for 25 or more wafers.
After the loadlock access door is closed and sealed, the loadlock chamber is then pumped by a pump system down to the vacuum level of the mainframe 198 before the slit valve is opened. The robot 194 which is mounted in the mainframe 198 then unloads the wafers from the cassette one at a time, transferring each wafer in turn to the first processing chamber. The robot 194 includes a robot hand or blade 206, which is moved underneath the wafer to be unloaded. The robot 194 then xe2x80x9cliftsxe2x80x9d the wafer from the wafer slot supports supporting the wafers in the cassette 190. By xe2x80x9clifting,xe2x80x9d it is meant that either the robot blade 206 is elevated or the cassette 190 is lowered by the handler mechanism 208 such that the wafer is lifted off the cassette wafer supports. The wafer may then be withdrawn from the cassette 190 through the passageway and transferred to the first processing chamber.
Once a wafer has completed its processing in the first processing chamber, that wafer is transferred to the next processing chamber (or back to a cassette) and the robot 194 unloads another wafer from the cassette 190 and transfers it to the first processing chamber. When a wafer has completed all the processing steps of the wafer processing system, the robot 194 returns the processed wafer back to the cassette 190 from which it came. Once all the wafers have been processed and returned to the cassette 190, the cassette in the loadlock chamber is removed and another full cassette of unprocessed wafers is reloaded. Alternatively, a loaded cassette may be placed in one loadlock, and an empty one in the other loadlock. Wafers are thus moved from the full cassette, processed, and then loaded into the (initially) empty cassette in the other loadlock. Once the initially empty cassette is full, the initially full cassette will be empty. The full xe2x80x9cprocessedxe2x80x9d cassette is exchanged for a full cassette of unprocessed wafers, and these are then picked from the cassette, processed, and returned to the other cassette. The movements of the robot 194 and the cassette handler 208 are controlled by an operator system controller 222 (FIG. 1), which is often implemented with a programmed workstation.
As shown in FIGS. 2 and 3, the wafers are typically very closely spaced in many wafer cassettes. For example, the spacing between adjacent wafers 230 and 232 in the cassette may be as small as 0.050 inches. Thus, the wafer blade 206 is often very thin, to fit between wafers as cassettes are loaded or unloaded. As a consequence, it is often preferred in many processing systems for the cassette and the cassette handler 208 to be precisely aligned with respect to the robot blade and wafer to avoid accidental contact between either the robot blade or the wafer carried by the blade and the walls of the cassette or with other wafers held within the cassette.
A metrology tool system which facilitates alignment of a cassette and a cassette handler is indicated generally at 400 in FIG. 4 and is described in copending application Ser. No. 09/294,301, filed Apr. 19, 1999 and entitled xe2x80x9cMETHOD AND APPARATUS FOR ALIGNING A CASSETTExe2x80x9d and assigned to the assignee of the present application. As described therein, the cassette alignment tool system 400 comprises a metrology tool or xe2x80x9ccassettexe2x80x9d 410 which emulates an actual cassette to be aligned. A cassette controller 412 is coupled by communication cables 414 to the metrology cassette 410, and a computer 416 is coupled by a communication cable 418 to the cassette controller 412. The metrology cassette 410 is secured to the cassette handler platform 200 in the same manner as an actual wafer cassette such as the cassette 190 of FIG. 2 and thus emulates the wafer cassette 190.
For example, the metrology cassette 410 preferably approximates the size and weight of a production wafer cassette full of wafers. In addition, the metrology cassette has alignment and registration surfaces similar to those of an actual cassette. The top surface of the top plate 612 and the bottom surface of the base plate 630 are both machined to imitate the bottom alignment and registration features of common wafer cassettes. This allows it to be inserted into most systems with measurement sensors directed upward or downward as needed.
Thus, the metrology cassette 410 has on the bottom of its frame a leading edge surface 422 and a trailing edge surface 424 of an H-bar 430 (FIGS. 5a-5c), and interior edge surfaces 562 of a pair of support runner or side rails 570 (FIG. 5b) which are received by corresponding alignment and registration surfaces of the cassette handler to align the cassette with respect to the handler. Similarly, the metrology cassette 410 has on the top of its frame an H-bar 430 (FIGS. 5c-5d), and side rails 570 (FIG. 5d) which are likewise received by corresponding alignment and registration surfaces of the cassette handler to align the cassette with respect to the handler in the inverted position. Still further, the metrology cassette 410 has rear edge surfaces 572 and side face surfaces 573 of a pair of rear guide rails 574 (FIGS. 5a and 5e) which are received by the handler. Variations and compromises from the features of individual cassettes can be made so as to accommodate the widest possible range of systems and cassettes. For example, by choosing the smallest size of the registration surfaces within the permitted range of tolerances of the cassettes to be emulated, the number of cassettes which can be emulated by a single tool 410 may be increased.
FIG. 6 shows an exploded view of an example of a typical cassette handler system which is indicated generally at 208. The handler system 208 is generally intended for use with cassettes which meet the SEMI (Semiconductor Equipment and Materials International, formerly known as Semiconductor Equipment and Materials Institute) standard for 150 mm cassettes. The SEMI standard is an internationally recognized standard which specifies many of the alignment and registration surfaces for 150 mm and other cassettes. The handler systems for non-SEMI standard cassettes are generally similar.
The handler system 208 includes an auto loader tilt out assembly 702 which facilitates automatic loading of cassettes into the handler system 208. The tilt out assembly 702 includes a receptacle 704 which is often referred to as the rear guide rail xe2x80x9cbucket.xe2x80x9d To install a cassette into the handler system 208, the bucket 704 is typically first tilted or rotated to a generally horizontal position using an auto loader tilt out mechanism 706 which is supported by an auto loader rotation support base 708. The alignment and registration surfaces of the rear guide rails of the cassette, such as the rear guide rails 574 of the metrology cassette 410, are received into a correspondingly shaped alignment and registration surfaces of a pocket 710 of the bucket 702 which provides an initial alignment of the cassette with respect to the handler system. The bottom of the cassette is placed against a bottom plate 712 of a platform 200 which includes a receptacle 714 often referred to as an xe2x80x9cHxe2x80x9d bar alignment nest.
When the auto loader tilt out assembly 702 is tilted forward, the cassette is rotated to an upright position in which the cassette is supported by the bottom plate 712 of the platform 200. The H bar alignment nest 714 has a slot 716 which has alignment and registration surfaces to receive the leading and trailing alignment and registration surfaces of an H bar of a cassette, such as the leading and trailing alignment and registration surfaces 422 and 424, respectively, of the H bar 430 of the metrology cassette 410 of FIG. 5a, to provide a more precise front to rear alignment of the cassette. In addition, the nest 714 is received between the cassette side rails 570 with the alignment and registration edges 718 of the nest engaging the alignment and registration interior edge surfaces 562 of the cassette side rails 570 to provide a more precise left to right alignment of the cassette.
As described in greater detail in the aforementioned copending application Ser. No. 09/294,301, filed Apr. 19, 1999, the metrology cassette 410 has one or more distance measurement devices 800 (FIG. 4) which can provide precise measurements of the position of a wafer or other workpiece being held by the robot blade within the metrology cassette 410. These wafer position measurements can be used to accurately align an actual wafer cassette such as the cassette 190 to the robot blade in such a manner as to reduce or eliminate accidental contact between the blade or the wafer held by the blade and the cassette or wafers held within the wafer cassette when the actual cassette is loaded into the cassette handler after completion of the alignment procedure and the metrology cassette is removed.
As best seen in FIGS. 5a and 7, the distance measurement device 800 of these figures includes three laser sensors A, B and C, each of which includes a laser head 810b, 810r or 810y, which is clamped in a mounting 812b, 812r or 812y, respectively, carried by the metrology cassette 410. These sensors operate based upon perpendicular beam, scattered reflection triangulation using a position sensing diode array. The light source (laser) impinges upon the target perpendicular to the surface of the target, preferably within a relatively small angle. The surface preferably provides a diffuse reflection that is visible to the sensing device over a relatively wide angle. The field of view of the sensing device is focused upon a linear optical sensor, the output of which is interpreted to determine the displacement of the target surface within the field of view. The geometry of the light path therefore forms a right triangle 522 with light from the light source traveling along the vertical edge and reflected light of the return path traveling along the diagonal. The distance between the sensor and the target may then be calculated using the Pythagorean theorem.
By precisely measuring the position of the wafer held by the robot blade, the orientation of the cassette handler platform may be adjusted to provide the desired alignment between the robot blade and the cassette held by the cassette handler system. To provide the desired adjustments, the handler system of FIG. 6 has a plurality of adjustment screws 830 which can be individually rotated to tilt an elevator attachment plate 832 and an elevator base plate assembly 834 which support the cassette handler platform 712 on which the cassette rests as described above.
Once a particular handling system has been properly aligned and calibrated to the robot blade and workpiece, the alignment of the handler may be clamped in place by tightening clamp screws 830 of the auto loader tilt out mechanism 706. The metrology cassette 410 may then be removed from the handler and processing of workpieces may begin using a standard workpiece cassette which was emulated by the metrology cassette 410. However, it is preferred that all handlers of a particular processing system be properly aligned prior to initiating processing of production workpieces.